The recovery boiler often constitutes a bottleneck for increased pulp production in kraft mills. The lime kiln in kraft mill chemical recovery plants is normally fired with fossil fuels such as fuel oil or natural gas. Firing lignin recovered from the black liquor in lime kilns have the double benefit of offloading the recovery boiler and replacing the lime kiln fuel with internal carbon dioxide neutral biofuel. Various methods for lignin precipitation and separation from black liquor have been proposed to offload recovery boilers. These methods have been met with limited commercial success partly because the recovered lignin often is contaminated with sodium and sulphur compounds and the market for unpure lignin is limited. The use of dried lignin powder as a fuel, for example in lime kilns has been proposed. Transport, storage and handling of lignin powder are hazardous and the material is easily ignited in oxygen or air. A lignin fuel which is in liquid or solid briquette form would be desirable and such lignin fuel is disclosed in Watkinson U.S. Pat. No. 5,478,366. The liquid lignin fuel of Watkinson however comprises 35 to 60% water and a significant amount of fuel oil. A high water content of a fuel will significantly lower its effective heating value and the portion of fuel oil in a lignin fuel decreases its value as carbon dioxide neutral biofuel.
A large portion of lignin is present in the spent cooking liquor produced during chemical pulping of lignocellulosic material. Chemical pulping typically involves reduction of wood to a fibrous mass through a series of cooking and bleaching operations using strong chemicals. The two principal chemical pulping methods are the alkaline kraft process and the acidic sulfite process, each producing a different type of extractable lignin. In the more common kraft process, wood chips are cooked in a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S). The alkaline attack causes fragmentation of lignin molecules, whose sodium salts are soluble at high pH. The spent cooking liquor, which is referred to as “black liquor”, is separated from the cellulosic fibres and typically sent to a chemical recovery plant wherein the organic components of the black liquor, including lignin, are combusted. Typical kraft black liquor contains around 40% alkali lignin, hemicelluloses including oligo- and monosaccharides from glucomannan and xylane and inorganic sodium and sulphur compounds.
Separation of lignin from the black liquor is generally not undertaken by the pulping industry due to the limited market for lignin material. However, lignin recovery techniques have been developed, and typically involve acidification of the black liquor with sulphuric acid (U.S. Pat. Nos. 4,111,928; 4,265,809; and 5,034,094).
Reduction in the pH of black liquor containing soluble lignin salts may be accomplished by introduction of acids which converts the phenolic hydroxyl groups on the lignin molecule, which are in ionized form, into their free phenolic or acidic form. This conversion renders the lignin insoluble in the black liquor and, as a result, it precipitates out. To precipitate the alkali lignin from the black liquor as water-insoluble products, the pH of black liquor initially having a pH around 13, is lowered to a pH of about 10.5 at which point the lignin begins to precipitate. Lignin precipitation can be further accentuated by reducing the pH level to about pH 2-3 with a strong mineral acid. The precipitated lignin can be washed with an aqueous solution to remove inorganic salts and other impurities. After drying the lignin it can be used as a fuel or as a precursor for fine chemicals preparation.
New alkaline pulping processes using low concentration of sulphur chemicals in the cooking liquor are under development and commercialisation. Such a process is described for example in U.S. Pat. No. 6,770,168. The spent cooking liquor from this and other sulphur free or low sulphur pulping processes contains dissolved lignin with low sulphur content and this lignin has a larger market potential than sulphur contaminated kraft lignin.